Exopolysaccharides from Rhizobium tropici Promote the Formation and Stability of Soil Aggregates: Insights from Soil Incubation

This study aimed to investigate the effects of r-type exopolysaccharides (EPSs) produced by the symbiotic bacteria Rhizobium tropici on soil aggregate formation and stability in loess sandy soil and to elucidate the independent and synergistic roles of EPSs in soil structure development. Experiments were conducted under both sterile and non-sterile soil conditions to distinguish the direct effects of EPSs from their interactions with indigenous soil microorganisms. Soil samples were treated with varying concentrations of EPSs and compared with untreated controls after undergoing a simulated weathering process. Aggregates were classified into four size fractions: <53 μm, 53–250 μm, 250–2000 μm, and 2000–5000 μm. Aggregate distribution and soil stability indicators, including the percentage of water-stable aggregates larger than 0.25 mm, mean weight diameter (MWD), geometric mean diameter (GMD), and fractal dimension (D), were analyzed. EPS application significantly promoted the formation of larger soil aggregates (>53 μm), with approximately 80% increases in the number of aggregates in the 53–250 μm and 2000–5000 μm fractions compared to the control. Soil stability was markedly enhanced, with a 41.7% increase in >0.25 mm water-stable aggregates, a 36.4% rise in MWD, and a 0.3% increase in GMD. The D decreased by 1.2% under 0.2‰ EPS treatment, indicating a more ordered soil structure. EPSs play a key role in promoting soil aggregate formation and enhancing soil stability. While microbial presence has a limited short-term effect on aggregation, the synergistic interaction between microorganisms and EPSs over time significantly enhances soil stability. This study provides new insights into understanding the independent and synergistic roles of EPSs in soil structure formation.